The downdrafting of hot gases from the combustion chamber of a traveling chain grate coking furnace through a series of airbox zones is well-known, this being generally done as the coal passes over the initial airbox zones adjacent the input end of the furnace to drive off all mositure and low temperature volatiles from the coal. The downdrafted gases may be fed to a utilization device or back into the combustion chamber of the furnace. See, for example, Whitten and Hagstrom U.S. Pat. No. 3,870,652. The handling of the downdrafted gases presents certain problems in that they are hot and laden with volatile material and furnaces of this type are notoriously leaky. Hence, any air entering the stream of downdrafted gases either at the airbox zones or downstream in the gas line or in the downdraft fans is likely to cause premature combustion.